Fibroblast growth factor 23 (FGF23) is secreted by osteocytes and regulates phosphate and vitamin D homeostasis. Serum FGF23 levels rise early in the course of chronic kidney disease (CKD), and high levels are independently associated with greater risks of CKD progression, cardiovascular disease and death. These findings have stimulated interest in designing therapies to lower FGF23 levels, but the approach is limited by poor understanding of the molecular mechanisms that stimulate FGF23 production beginning in early CKD. Iron deficiency is a novel stimulator of FGF23 transcription and may be a critical contributor to elevated FGF23 levels in CKD. Although overt iron deficiency is uncommon in early CKD, functional iron deficiency due to reticuloendothelial iron sequestration as a result of chronic inflammation is highly prevalent. In preliminary data presented in this application, we demonstrate that high levels of the iron sequestering protein, neutrophil gelatinase associated lipocalin (NGAL), reduce intracellular iron concentrations in osteocytes and stimulate FGF23 production. In further preliminary data, we report that NGAL-mediated reductions in cellular iron stabilize hypoxia inducible factor (HIF)1a, which initiates FGF23 transcription by binding the FGF23 promoter. Since NGAL expression is increased in response to kidney injury and inflammation, and serum levels are chronically elevated in CKD, we propose that kidney injury stimulates NGAL production, which induces iron efflux from osteocytes, creating intracellular iron deficiency that stabilizes HIF1a and promotes FGF23 production. In this innovative proposal, we will examine the regulatory effects of inflammation, NGAL, and true and functional iron deficiency on FGF23 production in health and in CKD. In Aim 1, we will define the regulatory effects of inflammation and NGAL on FGF23 production by exposing wild type and NGALko mice to three models of systemic inflammation, and we will show that endogenous and exogenous NGAL increases FGF23 production. In Aim 2, we will investigate HIF1a as a molecular mediator of FGF23 regulation that underlies the mechanistic crosstalk between NGAL, iron, inflammation and FGF23. We will perform CHIP assays and site-directed mutagenesis to test whether HIF1a binds the FGF23 promoter, and we will delete HIF1a in osteocytes to demonstrate that HIF1aOc-cko mice are resistant to NGAL and iron deficiency-induced increases in FGF23. In Aim 3, we will determine the clinical relevance and therapeutic potential of the NGAL and HIF1a pathways in CKD by studying the impact of NGAL and HIF1a deletion in the Col4a3ko mouse model of progressive CKD. We will investigate if Col4a3ko/NGALko and Col4a3ko/HIF1aOc-cko compound mutants display blunted increases in FGF23 production relative to Col4a3ko mice with intact NGAL and HIF1a systems, and we will also test the effects on FGF23 production of NGAL blocking antibodies and HIF1 inhibitors in Col4a3ko mice. The project will contribute new insights into the molecular regulation of FGF23 in health and in CKD, and support our ultimate goal of developing novel therapeutic approaches to improve outcomes in CKD.